The present invention relates to improvements made to the rapid cooling chambers of metal-strip heat treatment lines. Its purpose is more particularly to reduce the formation of wrinkles which form on metal strip subjected to cooling in continuous heat treatment lines, in which the said strip is made to pass through cooling zones provided with means for blowing a cooling
In order to clearly situate the technical field to which the present invention applies, reference will firstly be made to FIG. 1 which shows, schematically, in perspective and with partial cut-away, the cooling zone of a metal strip in a heat treatment line.
This FIG. 1 shows the strip 1 passing through the rapid cooling zone 2, by passing over an entry roller 3 and an exit roller 10. During the passage through the zone 2, the strip 1 is exposed to jets of cooling gas blown by a certain number of pairs of boxes, such as 4 and 5 and 6 and 7, each box being provided with blowing means and being positioned on either side of the strip. The cooling boxes, such as 4 and 5 and 6 and 7, have a limited length so as to allow one or more rollers or pairs of stabilizing rollers, such as the rollers 8 and 9, to be fitted between two consecutive boxes, such as 4 and 6 and 5 and 7 respectively, these well-known rollers being intended to guide and stabilize the strip 1.
The cooling gas is blown onto the strip by any conventional means such as those described for instance in U.S. Pat. No. 3 068 586.
The graph illustrated by FIG. 1A, associated with FIG. 1, shows the intensity xcfx86 of the cooling undergone by the strip 1 during its passage through the zone 2. During its entry between the first cooling boxes 4 and 5, the strip is suddenly exposed to a high cooling flux, the intensity of which remains constant over the entire length of the cooling box, then this intensity increases suddenly on leaving the said boxes. This variation in the intensity of the cooling undergone by the strip is repeated when it passes between each successive pair of cooling boxes placed over the entire length of the zone 2, as may be seen clearly in FIG. 1A.
The intensity of the strip cooling over the length of a box depends on the temperature of the cooling gas blown, on the geometrical characteristics of the blowing orifices of the boxes and on the distance of the strip from these orifices.
The performance of the strip-coating or heat-treatment lines is increased by the use of rapid cooling cycles or cycles comprising a succession of relatively rapid cooling slopes which require very high cooling gas flow rates to be used.
FIG. 2 of the appended drawings illustrates such a type of cooling cycle for which, for example, the strip is cooled according to the slopes A-B, C-D and E-F, at least one of these slopes being greater than the characteristic cooling slopes of the prior art. In FIG. 2, the sections B-C and D-E correspond to the discontinuities in the cooling which are associated with the gaps between the blowing boxes in order to fit the stabilizing rollers, such as the rollers 8 and 9 shown in FIG. 1.
This increase in the cooling slopes has given rise to a critical problem in this type of cooling zone, namely the formation on the strip of wrinkles which degrade the quality of the product. The objective of the present invention is to solve this problem by providing a solution which makes it possible to limit the formation of wrinkles on the strip during rapid cooling, while at the same time preserving the nominal speed of the strip in its passage through the rapid cooling zone, that is to say without any loss of production.